The invention falls in the technical area of the fabrication of components, can bodies and ends, easy-open or not, intended for the packaging of food products by drawing, in one or several passes, starting from a layered metal-plastic construction.
More precisely, the metal-plastic construction used in the present invention is of the type metal-polymer-metal, that is in which a layer of polymer is interposed between two metal sheets to which it is adhered.
In the text of this patent application, we will use without distinction for this concentration the terms metal-plastic-metal, metal-polymer-metal, or more simply in an abbreviated fashion, MPM.
There are numerous documents describing layered metal-plastic constructions. The majority of these concern metal-polymer or poly-metal-polymer constructions, metal-polymer-metal constructions being rarer.
Of those documents which do mention MPM constructions, there is none that teaches what range of materials and thicknesses are suitable for making drawn and ironed beverage or food cans.
By way of illustration one can cite the international PCT application filed on Jun. 25, 1981 by Metal Box Limited and published under the number WO 82/00020 on Jan. 7, 1982. This application has given birth in particular to the European patent EP 055 719.
This patent describes a metal-plastic construction in the simplest form of realization of a polyethylene (called for abbreviation PE) film attached to a foil or plate of metal. Another method of realization consists of two films of PE attached to opposite surfaces of a metal plate to form a complex PE-metal-PE. Finally, a third method of realization consists of two plates or foils of metals attached to opposite surfaces of a PE film. The PE used, obtained by copolymerization under a low pressure of ethylene and of butene-1, is of the type linear low density at a density of between 0.91 and 0.94 and it has been found that this particular type, whose characteristics are described in the application, has the interesting property of adhering directly to the metal without needing to use an adhesive. It suffices to adhere it to the metal by the simultaneous application of heat and pressure (heat sealing).
The metallic substrate can be steel, steel having a coating of tin or of chrome or of chrome/oxide or of zinc, of aluminum treated or not with nickel, copper, or zinc. It can have undergone a chemical conversion treatment.
Although there is no explicit teaching as to the desired absolute or relative examples of simple metal-plastic constructions in which the films of different types of polyethylene of 100 microns thickness are thus heat sealed on the plates of different metals; steel, tin plated steel, steel coated with chrome-chrome oxide, or aluminum of a thickness of 210 microns. The specimens obtained are then formed into hollow articles by folding, stamping, drawing, wall-ironing. The adhesion of the coatings is compared and demonstrates the superiority of linear low density polyethylene.
The French patent FR2 665 887 (Pechiney Emballage Alimemntaire) describes a capsule to fit over a cork made by drawing, drawing and ironing, or flow turning, characterized in that it is comprised of two layers of aluminum bound together by an adhesive layer of shore hardness less than 80. The adhesive layer can be constituted of an ethylene acrylic acid or of polyethylene, or of polypropylene modified with acid functionaltiy. The total thickness of the complex is comprised between 120 and 400 microns with the following percentage distribution of the total thickness:
______________________________________ Outer layer of aluminum 20 to 50% Adhesive layer 3 to 30% Inner layer of aluminum 40 to 60% ______________________________________
European Patent Application EP-A-0 046 444 assigned to Schwerzerische Aluminum AG describes an MPM composite foil in which the plastic layer could be as thick as the two metal layers combined. One stated requirement for achieving deep drawability is to select the plastic core layer and the metal surface layers such that when the composite is elongated, the load borne by the plastic core is greater than that borne by each of the metal strips. This condition is achieved by use of an oriented or stretched plastic layer. It is also stated that the soft or half-hard aluminum thin strip is particularly well suited. There is no teaching of drawing rigid containers such as cans.
A similar approach to MPM constructions is described in European Patent Application EP-A-O 034 781 which is assigned to BASF Aktiengesellschaft. The inventors of that application roll the plastic film before combining it with the metal foil in order to give the plastic ductility properties which are more like selected metals.
A metal-plastic-metal structural laminate which can be formed into various useful articles is described in European Application EP-A-0134958 assigned to Dow Chemical Company. This invention is in pat defined by a very wide range of thicknesses of the individual layers, of total thicknesses, and of ratio of thicknesses. It is further defined in terms of an ability of the laminate to withstand at least a certain level of stretch formability as measured in a standard laboratory test, the ability to be bent to a given sharp radius without metal rupture, and a certain level of thermal stability.
The patent does not contain any reference to any drawn or drawn and ironed shape nor to the ability of these laminate constructions to be formed by either a drawing process or by a drawing an ironing process. The laboratory test described is a form of biaxial tension test in which the material is uniformly stretched while maintaining the periphery, fixed in such a way as to thin the material. While such a stretch forming process is conventionally used in forming shallow parts such as automotive panels, it is not used for making food or beverage cans. A conventional drawing process such as is used to make food cans or for the initial steps to make beverage cans allows the material to flow from the periphery and results in little or no thickness reduction.
U.S. Pat. No. 3,298,559, assigned to Continental Can Company, describes laminated metal-plastic containers, such as cake pans, which are cold drawn in conventional forming dies. Among the metal-plastic containers described are some which are of the MPM type. Although wide ranges of thicknesses of metal and of plastic are claimed, there is no teaching as to the importance of the ratio of these thicknesses. Those examples which cover MPM constructions have a ratio of plastic to total metal thickness of between 5 and 9. The metal layers in the MPM examples are described as dead-soft or zero temper. There is no indication that such containers can be formed by a drawing and ironing process or that drawn containers suitable for food cans can be made.